Conventionally, heat exchangers used for various kinds of cooling systems and the like have been shown as examples. As such a heat exchanger, for example, there is exemplified a heat exchanger comprising a plurality of long plates disposed substantially parallel to one another and slits disposed between the long plates wherein a plurality of boards provided with concaves so as to be continuous in the longitudinal direction on the surfaces of some of the long plates, are laminated, the long plates of the adjoining boards are connected together to form tubes, the concaves form inside-tube flow passages and the slits form outside-tube flow passages (for example, see Patent Literature 1).